Now I have some audio issues
  • My 4PM is together and looks great but… When I turn it on, the current patch displays as expected. When I turn the encoder, I get a loud bang in the audio output, but there is no audio when I send it a MIDI note. The Shruthi is getting the MIDI value ‘though, it’s registering on the LCD. Also, if I press any of the buttons, the values are changing all by themselves, without my touching anything after the initial button press.

    Sounds like a short, but everything looks okay. Any ideas where I should start looking?



  • What filter do you have attached and what is the filter that you have selected in the system menu?

  • It looks like there are two problems here:

    • Something on the digital board (values changing all by themselves)
    • Something on the filter board (no sound)

    It might help if you troubleshoot both sides of the problem with another Shruthi-1 (4PM digital board with another filter board proved to work ; 4PM filter board with another digital board proved to work).

    Try to do that and let us know what happens.

    Some observations:

    • the filter board setting must be set to “4pm”.
    • the cv. setting must be set to “4cv” (if you set it to “programmer”, you get random values for all parameters when no programmer is attached)

  • Changing the CV setting to 4vc made a big difference. I have sound and no more random LCD light show! The output is really crunchy ‘though, although I kinda like it, but I’m not sure if it’s supposed to be that way. My son loves the sound ‘though.


  • Filter board set to “4pm”?

  • Host a .wav file of “init” somewhere, with some cutoff tweaking and resonance tweaking?

  • Okay, I’ll try and do that. Thanks again.

  • I don’t have an “init” patch, just “user” patches, but I recorded a quick sample of junon using the “normal” Shruthi followed by the sam patch on the 4PM. It sounds great on both but I just want to make sure the second is supposed to sound that dirty.


  • Your levels seem to be very high Randy. Maybe that’s part of the problem? I’m either not getting a waveform on soundcloud, or your levels were so high it clipped off the tops and made the track a solid block!

    Otherwise it sound okay to me.

  • I noticed that on Soundcloud when I clicked the link, but it wasn’t like that when I uploaded it. The levels on my own system were fine, maybe even a bit low. Most of the patches have much more bite to them, if that’s what it is supposed to sound like, great, I like it. I just wanted to make sure I didn’t screw up anything else.

  • Might be soundcloud having issues. Does it sound distorted to you in your studio?

  • I really don’t know. There’s lots of different kinds of distortion so I’m not sure. The 4PM is much dirtier than the original Shruthi, much more aggressive. Using the e-piano patch for example, if I change the filter mode to LP2, it’s back to being quite a bit mellower, the distortion disappears and the level is much quieter, so maybe it is just a level thing. Is there a way to equalize some of the levels or is the 4-pole LP in the 4PM just really aggressive?

  • I uploaded the .wav file instead:


  • The factory patches are optimized for the SMR-4… So any of the other filters are liable to produce odd results. You have to tweak them…

  • You are most probably correct. This thing is really, dare I say, gnarly sounding! Amazing bass, amazing leads.

  • I posted the same question a while back too…

    • Doesn’t sound at all the way it should
    • I am confused as to what you call a “2-pole” Shruthi.
    • Soundcloud always converts to shitty mp3. If you want to upload a .wav, soundcloud is not the place.
    • By default (4-pole + liquid), The 4PM shouldn’t sound much different from the SMR4 mkII. Not exactly the same but same territory. You can hear “Junon” on the 4PM here

    To me it sounds like a recording level problem ; and / or swapped resistor values in an amplifier stage.

  • It’s not a recording level problem, but it could be a resistor problem, I suppose I’ll need to check that.

  • I checked all of the resistors on the filter board and everything appears to be where it belongs. I thought perhaps Mouser had sent me 3.3K instead of 33K resistors, the colours are a bit hard to make out, but I removed one and tested it and got 32.8K, close enough I think.

    Is there a specific spot on the board that you think I should check?

  • Probe the signal at those points (sleeve of a jack to ground, tip of a jack to the point to probe)

    These are internal filter points ; so the signal won’t have any VCA applied to it, but it should sound clean…

  • 2, 3 and 4 sound the same, no distortion. Point 1 has more growl, sounds like there is a slightly de-tuned sub-oscillator. Kinda nice but not the sort of thing you want all the time.

  • Hmm, could you make a recording and host the .wav file somewhere?

  • Will do. Other than Soundcloud, where else can you host a .wav file?

  • I used Soundcloud for this one but I’ll check out Dropbox. This has 8 hits of a low G. The first four are from point 4 (which sounds the same as 2 and 3) and the last four are point 1.

    Another sample

    I did not alter the recording levels for the two.

  • OK. The filter core works. The culprits are thus the pole mixing circuit, the VCA, or the final amp.

    Next point to probe is point 8 of IC2 ; Junon preset, with the filter set to “lp4” and “liquid”.

  • This would probably be easier if I had a ‘scope. Glad we’re narrowing it down a bit ‘though, thanks for all of the help. When you say “point 8”, I’m assuming that’s pin 8?

  • So the problem is at the pole mixing stage. Could you post a photo of your assembly around IC2?

    There might be a bad solder point near IC2, or incorrect resistor values.

  • It seems to me that the 68R (above IC1) and 68k (above IC8) resistors have been swapped.

    Would explain AWFUL distortion in LP4 mode, since the gain would be 22000 / 68 = 323 instead of 22000 / 68000 = 0.323 . The other modes should be working OK na?

    Another thing: while resistors are not polarized, it’s always better to orient them consistently so that we can read their value at a glance (even better if they are aligned in the same direction as the pics on the site – I just have to zip through the picture to find the mistakes!).

  • Good advice, thanks. I’ll have a look at those two, you have really good eyes! I’m still having a terrible time trying to read the 5-band resistor codes so I stuck to whatever was written on the bags from Mouser. I could easily have swapped those two.

  • I must have built 18 of those, so I sort of know it by heart :)

  • How can you tell which way to read the resistors from? Which is band 5 and which is band 1, because sometimes the codes work in reverse too. Oh, and what are you doing awake so late, isn’t it way past bedtime over there?

  • The gap between the first 4 rings and the last one (which indicates the tolerance) is wider on some. Also, at some rare exceptions, if you read a resistor backwards, it rarely gives a value that makes sense according to the E12 / E24 series, or just to the BOM of the product!

  • It works! Maybe I should whisper that just in case, but so far it sounds like it should. Thanks alot! Can you do one, hopefully last (at least for this ‘round) favour for me? Can you post a sample of patch 39, e-piano?